The paper describes aspects of the statistical content knowledge of 46 preservice elementary school teachers. The preservice teachers responded to a written item designed to assess their knowledge of mean, median, and mode. The data produced in response to the written item were examined in light of the Structure of the Observed Learning Outcome (SOLO) Taxonomy (Biggs & Collis, 1982) and Ma's (1999 conception of Profound Understanding of Fundamental Mathematics (PUFM). Four levels of thinking in regard to comparing and contrasting mean, median, and mode are described. Several different categories of written definitions for each measure of central tendency are also described. Connections to previous statistical thinking literature are discussed, implications for teacher education are given, and directions for further research are suggested.Preservice Elementary Teachers' Conceptual and Procedural Knowledge of Mean, Median, and Mode Shulman (1987) used the term "content knowledge" to describe knowledge of the structure and methods of discourse within any given discipline. He stated that the many aspects of content knowledge "are properly understood as a central feature of the knowledge base of teaching" (p. 9). Research illustrates that attaining content knowledge is an important part of a mathematics teacher's development (Putnam, Heaton, Prawat, & Remillard, 1992). At the same time, research illustrates that teachers sometimes lack the content knowledge they need in order to teach elementary school mathematics (Ma, 1999). Ball, Lubienski, and Mewborn (2001) argued that Without such (mathematical) knowledge, teachers lack resources necessary for solving central problems in their workfor instance, using curriculum materials judiciously, choosing and using representations and tools, skillfully interpreting and responding to their students' work, and designing useful homework assignments (p. 433).The statistical measures of mean, median, and mode comprise a fundamental portion of the content knowledge elementary teachers need to possess. Curriculum documents around the world recommend that students develop some understanding of mean, median, and mode before entering secondary school (Australian Education Council, 1994; School Curriculum and Assessment Authority & Curriculum and Assessment Authority for Wales, 1996; National Council of Teachers of Mathematics, 2000). The National Council of Teachers of Mathematics (2000), for example, recommended that students should develop the abilities to "select and use appropriate statistical methods to analyze data" (p. 178) throughout Grades PreK-8. This included attaining proficiency in determining and applying the mean, median, and mode by the end of Grade 8.